Search

The existence and effect of different rare earth $(R^{2+/3+/IV})$ ions in $SrAl_{2}O_{4}:Eu^{2+},R^{3+}$ and $M_{2}MgSi_{2}O_{7}:Eu^{2+},R^{3+}$ (M: Sr, Ba) persistent luminescence materials was studied with XANES (x-ray absorption near edge structure) measurements at HASYLAB/DESY (Hamburg, Germany) and MAX-lab (Lund, Sweden). The experiments were carried out at 298 K for selected rare earth (co-)dopants $(Eu^{2+}; Ce^{3+}, Nd^{3+}, Sm^{3+}, Dy^{3+}$ and $Yb^{3+}$). The co-existence of $Eu^{2+}$ and $Eu^{3+}$ was observed in all materials. The co-dopants were always in the trivalent form.

In YMn2O5 the Mn atoms occupy two non-equivalent Wyckoff sites within the unit cell exhibiting different oxygen coordinations, i. e. the system can be characterized as a mixed-valence compound. For the formation of the orthorhombic crystal structure Jahn-Teller distortions are assumed to play an important role. In this study, we aimed at the investigation of the crystal structure on the substitution of Mn by the non-Jahn-Teller cation Fe+3. Therefore, we synthesized a series of YMn(2-x)FexO5 powder samples with x = 0; 0:5; 1 by a citrate technique. We utilized extended x-ray absorption fine structure (EXAFS), x-ray absorption near edge structure (XANES) analysis and Density Functional Theory to investigate the two non-equivalent Wyckoff sites within the orthorhombic crystal structure (confirmed for all compositions) occupied by transition-metal atoms. For quantitative determination of structural short-range order firstly all plausible options of substitution of Fe for Mn were discussed. On basis of these evaluations the EXAFS and XANES behavior was analyzed and appropriate crystallographic weights were assigned to the subset of structural models in accordance with the experimental data. From EXAFS analysis, using multiple scattering theory, we concluded only the 4h Wyckoff site to be occupied by Fe (occupancy refined was (100 +- 3)% in case of x = 1). Furthermore, taking the XANES spectra into account, we could verify the EXAFS results and additionally explain the differences in the Mn-K XANES spectra in dependence on x to be caused by changes of the dipole transitions to 4p final states. Since only one Wyckoff site is involved the experimentally observed limit to a maximum amount of x = 1 is explained. Additionally, a possible disorder, discussed in the literature, was not proven for our samples. With DFT calculations the experimental findings were verified on base of the total energy of the different possible electronic configurations. Crystal field effects were identified to be responsible for the site selective substitution of Fe for Mn.

A systematic and detailed investigation of the structure of As(x)Te(100-x) glasses (20

A double‐crystal, UHV‐compatible monochromator for XAFS applications at bending magnet beamlines has been designed. It uses two crystal sets, Si(111) and (311), on a common central rotation axis driven by an ex‐vacuo goniometer. All mechanical and electrical components are mounted on a 400 mm UHV flange which is attached to a compact vacuum chamber. The first crystals are water cooled using connector‐ and bellowless tubing through the fluidic sealed feedthrough of the central rotation. The first crystal set is mounted off‐axis and can be translated vertically to keep the fixed exit condition. The second crystal set uses small crystals of the same size as the first. In order to accept the reflected beam of the first crystal at small Bragg angles, it is tangentially translated along the beam. The angle can be varied from 5° to 55.5° resulting in a total energy range 2.4 – 43.4 keV for Si(111)/(311). Crystal sets are interchangeable by translating the vacuum chamber. Angle encoding is achieved by a Renishaw incremental optical encoder in vacuo.

We describe an inexpensive beam monitor for hard X-ray synchrotron applications which has good spectroscopic abilities and can operate without cooling. The device is centred on an inexpensive, commercial off-the-shelf, large area (1.2 × 1.2 mm2) Si photodiode operated in single counting mode. Measurements carried out at the HASYLAB synchrotron research facility have shown that it is fully spectroscopic across the energy range 8 keV to 100 keV with a measured energy resolution of ~ 1.2 keV FWHM at room temperature. The measured resolutions were found to be the same under pencil-beam and full-area illumination, indicating uniform crystallinity and stoichiometry of the bulk. The low cost, simplicity and performance of the detector make it suitable for a wider range of applications, e.g., in undergraduate laboratory experiments.